This invention relates generally to apparatus for recovering data and clock signals from optical disks and, more particularly, to such apparatus that recover data and clock signals simultaneously.
Apparatus of this particular kind is useful in recovering data signals recorded in the form of data pits or marks in a recording layer of an optical disk, wherein the recording layer incorporates a spiral or concentric servo track that is a permanent physical feature of the recording layer and that provides a track-following reference and defines the path along which data is written. In some types of prerecorded optical disks, such as read-only memories (ROM) disks, the data marks themselves function as the servo track. In other types of optical disks, only a servo track is prerecorded, and data marks are recorded on top of that servo track.
Apparatus for recovering recorded data signals from an optical disk of this kind typically incorporates an optical assembly that focuses a laser beam onto the servo track as the disk is rotated. This produces a reflected beam having an intensity that is modulated by the recorded pattern of data marks. The optical assembly directs this reflected beam to a detector, to produce a data signal representative of the recorded data signal. The optical assembly further incorporates a focus positioner for maintaining the incident laser beam properly focused on the servo track, and it further incorporates a tracking positioner for maintaining the beam aligned with the track in a cross-track direction as the disk is rotated.
One prior apparatus of this kind minimizes the degree by which the prerecorded clock signal interferes with recovery of the data signal by using a clock signal having a frequency higher than the highest frequency component of the recorded data signal. The recorded data signal is recovered from the optical disk using a first laser beam, having a wavelength selected such that its focused spot size is too large to effectively detect the recorded clock signal. Generally, the spot size of a focused laser mean is diffraction-limited to a diameter approximately equal to the beam""s wavelength. The recorded clock signal is recovered using a second laser beam, having a shorter wavelength and thus a smaller focused spot size. Both laser beams are focused onto the rotating disk using a common objective lens.
Although the apparatus described briefly above is effective in recovering both a data signal and a clock signal from an optical disk, it is considered to be unduly complex. Besides requiring the use of two separate lasers, with associated optics, the apparatus also requires the use of a dichroic beamsplitter for first merging the two incident laser beams together and then separating the two reflected beams from each other.
It should, therefore, be appreciated that there is a need for an improved apparatus for recovering data and clock signals simultaneously from an optical recording medium such as a rotatable disk, while avoiding the need for two separate lasers, yet preventing the recorded clock signal from interfering with the recovery of the data signal. The present invention fulfills this need and provides further related advantages.
The present invention is embodied in any improved apparatus for recovering data and clock signals simultaneously from an optical recording medium, while avoiding the need for two separate lasers, yet preventing the recorded clock signal, which has a frequency higher than the highest frequency components of the recorded data signal, from interfering with the recovery of the data signal. More particularly, the apparatus incorporates a laser configured to generate an incident beam of light having a predetermined nominal wavelength, and an optical assembly configured to focus the incident beam of light onto the optical recording medium. This produces a reflected beam having an intensity that is modulated according to the recorded data and clock signals. A beam splitter separates the reflected beam into first and second beams. A nonconfocal data detector receives the first beam and produces a recovered data signal that varies according to the recorded data signal, wherein the bandwidth of the nonconfocal data detector is insufficient to detect the amplitude modulation of the recorded clock signal. Finally, a confocal clock detector receives the second beam and produces a recovered clock signal representative of the recorded clock signal.
In more detailed features of the invention, the optical assembly includes a single objective lens configured to focus the incident beam onto optical recording medium and to collect light reflected therefrom, to form the intensity-modulated reflected beam. The reflected beam retraces the path of the incident beam back to a polarizing beam splitter, for separation. The confocal clock detector comprises a pinhole located at a the conjugate plane with respect to the laser and the focused beam of light on the optical recording medium.
The optical recording medium preferably takes the form of a rotatable disk incorporating a recording layer having a servo track that records the data and clock signals. In addition, a focus error detector receives a portion of the reflected beam, for use in positioning the objective lens such that the incident beam is properly focused on the disk""s recording layer. Further, a track error detector likewise receives a portion of the reflected beam, for use in positioning the objective lens such that the incident beam properly tracks the servo track formed in the disk""s recording layer.
Other features and advantages of the present invention should become apparent from the following description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention.